Boiling Point And Pressure: What, How, Relationship, Effects And Detailed Facts


This article discusses about the relation between boiling point and pressure. A misconception lies among us that the boiling point is related to temperature only. But primarily it is the pressure which is responsible for the boiling to take place.

The boiling of liquid starts when the vapour pressure of the liquid starts touching the value of ambient or surroundings pressure. The pressure at which boiling takes place is called as saturation pressure. Pressure of the liquid keeps on increasing with increasing temperature. We shall study more about this relationship in this article.

What is partial pressure?

In simple words partial pressure is defined as the pressure exerted by a certain type of liquid molecules in a mixture.

This pressure is the exact same pressure that the liquid molecules must have exerted if these were the only molecules occupying the whole volume. Partial pressure term comes into play when there are more than one type of liquid molecules present in the system.

What is vapour pressure?

Vapour pressure is the pressure exerted by the liquid molecules when they are about to change into gaseous molecules.

The vapour pressure increases with temperature. The surface of the liquid starts boiling when the surface vapour pressure is equal to the ambient pressure. The atmosphere exerts some pressure on the liquid, when this value of atmospheric pressure is reached by vapour pressure, boiling starts taking place. 

How does pressure affect boiling point?

Pressure, as discussed above, is one of the most important factor that affects directly the boiling point of any liquid.

For the surroundings, if the surrounding pressure is very high, it will take more time and more heat for the liquid to reach the ambient pressure value and hence the boiling point of the liquid will be more. When the ambient pressure is low then the liquid will reach the ambient pressure value soon. In this case, the boiling point will be lower as compared to boiling point at surroundings with high pressure.

Boiling point and pressure relationship

Now we have a clear view of how pressure affects the value of boiling point. The main factor affecting the boiling point is atmospheric pressure. (Although temperature also plays an active role)

We shall focus our discussion on relation between pressure and boiling point only. Lets us assume that the heat transfer rate is constant. This way the liquid molecules will slowly get heated up resulting in increase in their vapour pressure. As the vapour touches the ambient pressure, the liquid will start boiling.

boiling point and pressure
Image: Boiling

Image credits: user:Markus SchweissKochendes wasser02CC BY-SA 3.0

Boiling point and pressure equation

The equation which draws the relationship between boiling point and pressure numerically is called as Clausius-Clapeyron equation.

The Clausius-Clapeyron equation is given below-

Does boiling point increase with pressure?

As and when the pressure of the ambient increases, the boiling point also increases but only upto the critical point.

It is the point where the properties of both the phases, that is, liquid and gases, are exhibited by the substance. We shall more about critical point in later sections of this article. Also, we can note this fact that the boiling point of the liquid decreases when the pressure drops until the triple point is reached.

How does boiling point increase with pressure?

The boiling point can be defined as the value of temperature at which the vapour pressure of liquid becomes equal to the ambient pressure. This value of temperature depends on the ambient pressure.

If the ambient pressure is more, it takes more time for the liquid’s vapour pressure to reach the value ambient pressure. The heat source constantly supplies heat to the liquid to increase its pressure. When the pressure at the outside is more then the boiling time will also be more.

How does water boil at low pressure?

At low pressures, the water starts boiling in a very short time. This is because the time required for the vapour pressure to reach the value of ambient pressure is less.

A notable example is that the food is cooked very fast on mountains. This is because the ambient pressure is very less compared to the pressure at sea level. The liquid takes more time to boil when it is at sea level. 

Does water boil faster at high or low pressure?

We have discussed rigorously about the effect of pressure on boiling point. It is made to us very clear that the water will start boiling faster at lower pressures.

The reason being the same as discussed above. That is, the time required for the vapour pressure to reach the value of ambient pressure is less. The water starts boiling when the vapour pressure reaches the value of ambient pressure.

Factors affecting boiling point

We have discussed a lot of things about what affects the boiling point. Let us get a more detailed view on these factors.

The factors affecting the boiling point are given below-

  • Temperature– Temperature is responsible for increasing or decreasing the vapour pressure of the liquid. When the temperature is high, the vapour pressure increases and likewise, when the pressure is low, the vapour pressure decreases.
  • Vapour pressure– When the vapour pressure reaches the value of ambient pressure, the liquid starts boiling. The pressure exeerted by the liquid when it is about to change its phase is called as vapour phase.  
  • Atmospheric pressure– As the name suggests, the atmospheric pressure is the pressure exerted by atmosphere. Atmospheric pressure plays a very important role in determining the boiling point of the substance. If the pressureexerted by atmosphere is less then the boiling point will also be less and on the other hand the boiling point will be more if the pressure exerted by the atmosphere is more.

What is critical point?

Simply put, critical point of a substance is that point at which the properties of liquid such as density, temperature, pressure etc are equal to that of its own gaseous state.

This can be said as an equilibrium state where the substance exists in both the liquid and gaseous phase. The molecules exhibit properties of both liquids and gases at the same time.

What happens at critical point?

When the temperature of the liquid is raised, the density of the liquid falls down and simultaneously the density of gas starts increasing.

When the densities of both liquid and gas become equal, then the particular point is called as critical point. Here both gas and liquid phase properties are exhibited by the substance.

Abhishek

Hi ....I am Abhishek Khambhata, have pursued B. Tech in Mechanical Engineering. Throughout four years of my engineering, I have designed and flown unmanned aerial vehicles. My forte is fluid mechanics and thermal engineering. My fourth-year project was based on the performance enhancement of unmanned aerial vehicles using solar technology. I would like to connect with like-minded people.

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